1. Field of the Invention
This invention relates to a semiconductor device and particularly, but not exclusively, to a semiconductor device that emits visible radiation in the wavelength range 630 nm to 680 nm, such as a semiconductor laser device or light-emitting diode. The device may be of the edge-emitting or of the surface-emitting type.
2. Description of the Related Art
Laser devices or laser diodes (LDs) fabricated in the (Al,Ga,In)P material system which emit visible light in the 630 nm-680 nm wavelength range are becoming increasingly important components of professional and consumer products. For example, it is envisaged that the Digital Video Disc (DVD) system will employ a 635 nm-650 nm wavelength LD capable of delivering up to 30 mW output power up to a temperature of 60xc2x0 C. The next generation of semiconductor lasers will need an even greater maximum power output up to a higher (eg. 70xc2x0 C.) operating temperature.
By the (Al,Ga,In)P system is meant the family of compounds having the general formula (AlxGalxe2x88x92x)lxe2x88x92yInyP, where both x and y are between 0 and 1. One particular advantage of this semiconductor system is that it is lattice-matched to a GaAs substrate when the indium mole fraction, y, is equal to 0.48.
A principal limitation of current (Al,Ga,In)P laser diodes is that they are incapable of operating for long periods (or with a sufficiently low threshold current) at the highest specified operating temperature. It is generally believed that this in caused by electron leakage from the active region of the device into the surrounding optical guiding region and subsequently into the p-type cladding region.
The generic structure of a separate confinement laser structure intended to generate light at 630-680 nm will now be described with reference to FIGS. 1(a), 1(b) and 1(c).
FIG. 1(a) is a schematic band structure of a separate confinement laser structure fabricated in the (Al,Ga,In)P system. It consists of an n-doped (Al0.7Ga0.3)0.52In0.48P cladding region 1, an (Al0.5Ga0.5)0.52In0.48P optical guiding region 2, 4, a GaInP quantum well active region 3 disposed within the (Al0.5Ga0.5)0.52In0.48P optical guiding region, and a p-doped (Al0.7Ga0.3)0.52In0.48P cladding region 5. A p-type contact layer (not shown in FIG. 1(a)) may be provided on the p-type cladding region 5, and an n-type contact layer (not shown) may be provided on the n-type cladding region 1. Optical transitions giving rise to laser action in the quantum well active region 3 of the laser diode originate from electrons in the xcex93-band in the GaInP quantum well active region.
The terms xcex93-band and X-band as used herein refer to symmetry points in the Brillouin zone and are standard terms in solid state physics, see for example R. A. Smith xe2x80x9cSemiconductorsxe2x80x9d, (Cambridge University Press, 1978). The terms xcex93-minimum and X-minimum refer to the minimum energy level of the xcex93-band and the X-band, respectively.
The minimum energy in the conduction band of (Al,Ga,In)P is a function of the aluminium content. There is a crossover from a xcex93-band minimum to an X-band minimum at an aluminium concentration of about 0.55.
The aluminium mole fraction of the cladding regions 1, 5 need not be 0.7, provided that it is sufficient to provide an effective potential barrier to confine xcex93-electrons in the optical guiding region. FIG. 1(b) illustrates a similar laser structure to that of FIG. 1(a), but the cladding regions 1, 5 are formed of AlInP rather than (Al0.7Ga0.3)0.52In0.48P in order to increase the potential barrier confining xcex93-electrons in the optical guiding region 2, 4.
In the laser structures shown in FIGS. 1(a) and 1(b) the active region 3 is a single GaInP quantum well layer. It is alternatively possible for the active region to contain two or more quantum well layers separated by barrier layers.
FIG. 1(c) shows the xcex93-conduction band and valence band of a (Al,Ga,In)P laser device in which the active region comprises a plurality of quantum well layers. In the embodiment shown in FIG. 1(c), the active region 3 comprises three GaInP quantum well layers 3a with each neighbouring pair of quantum well layers being separated by a barrier layer 3b. The barrier layers 3b are formed of a material having a higher xcex93-band than the material used to form the quantum well layers 3a, such as (Al0.3Ga0.7)0.52In0.48P or (Al0.5Ga0.5)0.52In0.48P. In the laser shown in FIG. 1(c) the barrier layers 3b are formed of (Al0.5Ga0.5)0.52In0.48P, the same material as used for the optical guiding region 2, 4.
Degradation of semiconductor lasers has been a major problem in developing commercial devices. AlGaAs/GaAs lasers having a lasing wavelength of around 0.85 xcexcm were initially developed in the 1970s, but early examples of these lasers degraded quickly during use and, as a result, had a low lifetime and were unsuitable for commercial applications. It took a considerable time to overcome the significant degradation problems involved with these lasers, but long lifetime AlGaAs/GaAs lasers are now commercially available. M. Fukuda reports, in xe2x80x9cReliability and Degradation of Semiconductor Lasers and LEDEsxe2x80x9d ISBN 0-89006-465-2, that AlGaAs/GaAs lasers having a life time greater than 10,000 hours are now commercially available.
In order for (Al,Ga,In)P lasers to be commercially successful, these lasers must have a life time comparable with that of AlGaAs/GaAs lasers.
At present, wide band-gap phosphide lasers operating in the visible spectrum at a wavelength of about 650 nm display a severe degradation problem. Although the lifetime of low power phosphide lasers is approximately 10,000 hours which is satisfactory for commercial purposes, a typical lifetime of a high-power phosphide laser is only about 5,000 hours which is not commercially acceptable. Furthermore, it is necessary to anneal the lasers in order to obtain these lifetimes, and lasers that are not annealed have much shorter lifetimes.
The degradation problem is particularly serious for lasers fabricated using molecular beam epitaxy (MBE). At present, phosphide laser structures that are grown by MBE have to be thermally annealed in order to improve their reliability and to decrease the threshold for laser operation. It Is presumed that the annealing process removes (or at least moves) some of the non-radiative recombination centres in the material. It is, however, undesirable to carry out an annealing step. One common p-type dopant for the p-type cladding region is beryllium, and if a beryllium-doped laser device is annealed beryllium can diffuse from the p-type cladding region into the active region. Such diffusion will degrade the performance of the laser device, and may also lower the yield of the manufacturing process.
One possible reason for the degradation of phosphide lasers is that the degradation is due to oxygen contamination of the active region. Oxygen contamination of aluminium-containing phosphide materials can easily occur, owing to the high reactivity of aluminium with oxygen. Oxygen introduces non-radiative defects into the active layer, and this may give rise to a high threshold for laser oscillation.
Oxygen is a common contaminant in aluminium-containing alloys, because of the high reactivity of aluminium with oxygen-containing species. In the growth of AlGaAs/GaAs semiconductor structures, it is known that oxygen atoms are sufficiently mobile during the epitaxial growth process for them to migrate to the interface and to become trapped at the interface. Oxygen forms non-radiative centres in GaAs and in AlGaAs, and so the presence of oxygen tends to reduce the performance and reliability of AlGaAs/GaAs lasers. N. Chand et al. report, in xe2x80x9cJ. Vac. Sci Technol. Bxe2x80x9d Vol. 10 (2), p807, 1992, that an AlGaAs layer having an aluminium mole fraction of 0.3 or greater has a typical oxygen atom density of at least 1017 cmxe2x88x922. In contrast, in GaAs the oxygen concentration is typically too small to detect.
Chand et al. show that oxygen tends to accumulate at the AlGaAa/GaAs interface, so giving rise to an xe2x80x9coxygen spikexe2x80x9d in the active region of the device. They also show that a beryllium-doped cladding layer has a greater oxygen concentration than a silicon-doped cladding region.
FIG. 3(a) shows the oxygen concentration of the laser of FIG. 1(c). The band structure of the laser is repeated in FIG. 3(b), so that the oxygen concentration can conveniently be related to the layers of the device.
The laser of FIG. 1(a) consists mainly of (Al,Ga,In)P with a non-zero aluminium content, so that its background oxygen concentration is expected to be approximately 1017 cmxe2x88x923. In the growth of this structure, oxygen migrates to the interfaces between the quaternary layers and the GaInP quantum well layers 3a. This gives rise to a localised high oxygen concentration, or xe2x80x9cspikexe2x80x9d, in the regions of each interface. These spikes in the oxygen concentration increase the oxygen concentration in the active layers, and lead to the formation of non-radiative traps in the quantum well active layers. These traps degrade the performance and reliability of the laser, as discussed above.
FIG. 4 shows the oxygen concentration for a conventional laser structure fabricated in the (Al,Ga,In)P system. The oxygen concentration was measured by secondary ion mass spectrometry (SIMS). FIG. 4 shows the results of two measurements, carried out at different scanning rates.
The sample measured has an active region containing 3 GaInP quantum well layers sandwiched between (Al0.5Ga0.5)0.52In0.48P barrier layers, an (Al0.5Ga0.5)0.52In0.48P waveguiding region, and an (Al0.7Ga0.3)0.52In0.48P cladding region. The cladding region and the waveguiding region each have an oxygen concentration measured by SIMS of around 1017 cmxe2x88x923, compared to an oxygen concentration as low as 1015 cmxe2x88x923 in the GaInP quantum well layers (which are aluminium-free). There is therefore a sharp spike in the oxygen concentration at each interface between a barrier layer and a quantum well layer (in a similar manner to the case of GaAlAs/GaAs). The SIMS instrument cannot resolve such sharp features, so that FIG. 4 in fact shows a convolution of the oxygen level across the three quantum well layers. The single observed spike in the oxygen concentration is an artefact of the measurement technique.
The (Al0.7Ga0.5)0.52In0.48P cladding layer and the (Al0.5 Ga0.5)0.52In0.48P optical guiding regions are measured as having oxygen concentrations of around 1017 cmxe2x88x923. The oxygen concentration in the active region is significantly greater than this. (As noted above, the single oxygen spike of FIG. 4 is an artefact of the measurement technique.)
The region at a depth of 2.6-2.8 xcexcm is a GaInP layer which, as anticipated, is observed as having a low oxygen concentration. The oxygen spike that occurs at a depth of 2.8-3.0 xcexcm is at the interface between the epilayer structure and the substrate, and is due to oxygen not having been completely removed from the substrate before the epitaxial growth process.
There have been a number of attempts to produce semiconductor laser devices having an aluminium-free active region. A. Al-Muhanna et al. have proposed, in xe2x80x9cApplied Physics Lettersxe2x80x9d Vol. 72, No. 6, pages 641-642 (1998), a laser device in the InGaAlP system with an InGaAsP active layer. This laser emits light having a wavelength of 730 nm. they report that this laser has an improved efficiency, and they attribute this to the use of an aluminium-free active region.
Wade et al. report, in xe2x80x9cApplied Physics Lettersxe2x80x9d Vol. 70, No. 2, pages 149-151 (1997), a laser emitting light at 830 nm and having an aluminium-free active region. This device again uses an InGaAsP layer as the active region, and this is disposed in an InGaP optical guiding region. Wade et al. report an improvement in the reliability of the laser, which is attributed to the use of an aluminium-free active region. They also report an improved COD (catastrophic optical mirror damage) power density level.
However, the approach of using a InGaAsP active layer cannot be used to obtain a laser device emitting light in the wavelength range 630 nm to 680 nm, because the band gap of InGaAsP is too small to produce emission in this wavelength range. This is the case for all III-V semiconductors (with the exception of the nitrides).
EP-A-0 476 689 discloses a laser device emitting in the 630-680 nm range in which the active region is formed of strained AlGaInP layers. The barrier layers are under tensile strain and the quantum well layers are under compressive strain and the different strains induce different band structures in the barrier and quantum well layers. This document also suggests that a similar approach could be used with an active region formed of strained GaInP layers, although the absence of aluminium would increase the emission wavelength of the laser.
The lasers suggested in EP-A-0 476 689 have an AlGaInP waveguide adjacent to the active region. Oxygen contamination will occur in the waveguide since it contains aluminium, and a spike in the oxygen concentration will occur at the interface between the active region and the waveguide.
U.S. Pat. No. 5,331,656 discloses a GaInP laser that emits in the 550-590 nm wavelength range. The active region contains quantum well layers and barrier layers both formed of GaInP doped with nitrogen. However, doping with nitrogen is difficult to carry out in practice. Moreover slight doping with nitrogen will not significantly change the alloy composition and so will not cause significant strain, whereas heavy doping with nitrogen will significantly reduce the bandgap.
Lasers having an active region in which the quantum well layers and barrier layers are oppositely strained are also disclosed in U.S. Pat. No. 5,903,587, JP-A-11 145 549 and U.S. Pat. No. 5,841,152. However, these documents do not relate to an (Al,Ga,In)P laser emitting in the 630-680 nm wavelength range. JP-11 145 549 discloses an MQW layer in which the quantum well layers and the barrier layers are both InGaAsP layers. The laser disclosed in U.S. Pat. No. 5,903,587 emits in the 0.98 to 1.02 xcexcm range and has an active region containing barrier layers formed of GaAsP and quantum well layers formed of GaInAs. U.S. Pat. No. 5,841,852 discloses a laser emitting light having a wavelength of 1.3 xcexcm, in which the barrier layers are InGaAsP, and the quantum well layers are InGaAs.
A first aspect of the present invention provides a semiconductor device having an aluminium-free active region, the active layer comprising one or more quantum well layers and one or more barrier layers, the quantum well layer and the barrier layer being disposed alternately; wherein the or each barrier layer is a strained layer; and wherein the device further comprises a first cladding layer and an aluminium-free layer disposed between the first cladding layer and the quantum well layer closest to the first cladding layer.
The term xe2x80x9caluninlum-free layerxe2x80x9d as used herein refers to a layer that does not have aluminium as an intentional constituent.
Straining a layer of semiconductor material can increase the band-gap of the material. The use of a strained layer for the barrier layer(s) provides a barrier layer or layers having the necessary xcex93-band energy to provide an effective barrier to the transport of xcex93-electrons between neighbouring quantum well layers without the need to intentionally introduce aluminium into the barrier layer(s). The invention thus makes possible a device having an aluminium-free active region.
Moreover, oxygen is likely to accumulate at the interface with the first cladding layer during the growth process, leading to a region of high oxygen concentration. Providing an aluminium-free layer between the first cladding layer and the quantum well layer closest to the first cladding layer spaces the active region from this region of high oxygen concentration, and reduces the effect of the region of high oxygen concentration on the operating properties of the device.
A second aspect of the present invention provides a semiconductor device having an aluminium-free active region, the active layer comprising one or more quantum well layers and one or more barrier layers, the quantum well layer and the barrier layer being disposed alternately; wherein the or each barrier layer is a strained layer; and wherein the device emits, in use, light having a wavelength in the range 630 to 680 nm.
As noted above, the use of a strained layer for the barrier layer(s) avoids the need to intentionally introduce aluminium into the barrier layer(s). The invention thus makes possible a device having an aluminium-free active region and that can emit light in the 630-680 nm wavelength range of the visible spectrum. By avoiding the need to introduce aluminium into the active region of, for example, a laser device, degradation of the device owing to oxygen contamination of the active region is reduced and the lifetime of the device is consequently increased.
A device according to the second aspect of the invention may further comprise: a first cladding layer; and an aluminium-free spacer layer disposed between the first cladding layer and the quantum well layer closest to the first cladding layer.
A device according to the first aspect of the invention may emit, in use, light having a wavelength in the range 630-680 nm.
A device according to either aspect of the invention may emit, in use, light having a wavelength in the range 635-630 nm.
The or each barrier layer maybe a tensile strained layer.
The lattice mismatch between the or each barrier layer and the or each quantum well layer may be small. This increases the critical thickness of the barrier layer(s).
The or each quantum well layer may be lattice matched to the cladding layer(s). Alternatively, the or each quantum well layer may be a compressive strained layer. This allows a device with a strain compensated active region to be produced, by using a compressive strained layer for the or each quantum well layer and a tensile strained layer for the or each barrier layer.
The or each barrier layer may contain indium. Introducing indium into the barrier layer(s) reduces the strain in the barrier layer(s) and so increases the critical thickness of the barrier layer(s).
The aluminium-free layer may be a strained layer.
The device may be fabricated in the (Al,Ga,In)P material system. This allows for example a device emitting light in the region 630-680 nm and having an aluminium-free active layer to be produced.
The or each active layer may be a GaxInlxe2x88x92xP layer and the or each barrier layer may be a GayInlxe2x88x92yP layer, where y greater than x. For example, the or each active layer may be a Ga0.52In0.48P layer and the or each barrier layer may be a Ga0.61In0.39P layer, or alternatively the or each active layer may be a Ga0.52In0.48P layer and the or each barrier layer may be a Ga0.71In0.29P layer. Tensile strained layers of Ga0.61In0.39P and Ga0.71In0.29P have approximately the same band-gap as (Al0.3Ga0.7)0.52In0.48P and (Al0.5Ga0.5)0.52In0.48P respectively, and so can be used to replace these materials without significantly affecting the band-gap profile.
Alternatively, the or each active layer may be a Ga0.43In0.57P layer. This material has a smaller lattice constant that GaAs, and so will form a compressive strained layer when grown on a layer that to lattice matched to GaAs.
The device may be a laser device. Alternatively, the device may be a light-emitting diode.